Voice, fax and data communication capabilities are available through mobile satellite communication systems. For example, the Inmarsat-M™ and Inmarsat mini-M™ systems support a data rate of 2.4 kbit/s, while the Inmarsat-B™ system provides data rates of up to 16 kbit/s. However, in terrestrial communications data rates of 28.8 kbit/s are commonly used over a PSTN under the ITU V.34 standard, and data rates of 56 or 64 kbit/s per channel are available over ISDN. Many internet-based and conferencing applications require the data rates available over terrestrial networks. Such applications cannot be used satisfactorily on conventional mobile satellite terminals.
Mobile satellite communication channels are subject to many different sources of noise as well as fading, particularly when the mobile terminal is moving. However, bit error rates of 10−6 or less are desirable if the service is to have performance comparable with terrestrial data communications, which limits the data rate operable on the satellite channel. The data can be encoded for error correction so as to reduce the bit error rate, but this also reduces the data rate.
Satellite communications typically involve much greater delay than terrestrial communications. As well as the propagation delay between an earth station and a satellite, delay is also incurred in formatting data into transmission frames and in encoding the data to provide error detection and correction. Complex coding and decoding algorithms can reduce the bit error rate of a satellite channel, but these algorithms generally involve buffering and intensive processing, which add to the delay. Excessive delay is inimical to real-time applications such as telephony and conferencing.
U.S. Pat. No. 5,568,483 describes a method for formatting data of different data rates for transmission over a transmission medium. European patent publication No. 0 676 875 A discloses a transmission method for wireless circuits such as satellite circuits, in which data is encoded at a variable rate depending on the priority of the data transmitted.
International patent publication No. WO 96/164492 discloses a wireless digital transmission technique in which pilot symbols are inserted periodically in a stream of data symbols.
According to one aspect of the present invention, there is provided a satellite communications technique in which a pilot symbol is transmitted after every 25 or 29 data symbols.
According to another aspect of the present invention, there is provided a method of transmitting both user data and in-band signaling information such that frames are transmitted containing either multiplexed user data and signalling information or multiplexed signaling information and dummy data, with the frame length being the same in either case.
According to another aspect of the present invention, there is provided a method of formatting user data, which is received in user data frames comprising four subframes each of equal length, into output frames each corresponding to an integral number of user data frames.